Conventional EM (electromagnetic) or DC (direct chill) aluminum casting typically involves controllably discharging molten aluminum to one or more continuous casting stations. Each casting station includes a concave bottom block and a surrounding mold. The mold can be either a DC mold or an EM mold. In either case, casting is performed by discharging aluminum onto the bottom block and by gradually lowering the bottom block while cooling the mold and the lower portions of aluminum.
Various factors are responsible for the quality of the resulting ingot. Casting procedures have become quite complex in order to ensure consistent quality from one casting to another. Generally, three variables must be closely monitored and controlled to achieve optimum results: molten metal level, drop rate, and cooling rate.
U.S. Pat. No. 4,498,521, to Takeda et al., describes a system for maintaining a constant level of molten metal in a plurality of vertically-oriented continuous or semicontinuous casting units. The Takeda system includes a float level sensor for precisely measuring the level or elevation of molten metal in a casting station during all portions of the casting process. The float level sensor is suspended from a molten metal launder over a casting station.
In conventional casting systems, a metal launder typically has a plurality of metal discharge spouts, and spans a like number of casting stations. It is generally impractical to provide external vertical support for the launder at any intermediate positions along its length. Rather, the launder is supported at both of its ends, and is designed with sufficient structural strength to support its own weight and the weight of any molten aluminum contained therein.
However, a metal level sensing system such as described by Takeda et al. requires not only that a launder be strong enough to support itself, but that it be strong enough to establish and maintain a stable measurement platform from which molten metal level measurements can be made. This is because all vertical measurements in the Takeda system are referenced from the launder beam, along its unsupported length over casting stations. Molten metal level is thus controlled and maintained relative to the elevation of the metal distribution launder. Any change in the elevation of the launder will cause a corresponding change in the level of molten aluminum within an underlying casting station. It is, therefore, extremely desirable to provide a metal distribution launder which resists movement or flexure in the vertical direction.
Vertical launder flexure results from two causes: variability in the amount and corresponding weight of contained molten aluminum, and vertical temperature gradients produced in structural parts of the metal distribution launder. Flexure from the first cause can be reduced by providing structural reinforcement along the length of the metal distribution launder. However, structural reinforcement is not effective against heat-induced launder flexure.
One cause of temperature gradients within a launder is radiant heat from underlying molten metal in casting stations. A more significant cause is heat transferred or conducted from molten metal contained within the launder itself. Since molten metal flows along the bottom of a distribution launder, resulting heat buildup in the structural members of the launder tends to be greater toward the bottom of the launder than toward the top of the launder. This causes the launder to bow. In some cases such bowing can be quite extreme, virtually eliminating any chance of accurately measuring the elevation of molten metal within underlying casting stations.
Previous attempts to eliminate launder bowing and flexing have involved structural reinforcements and launder cooling. However, these attempts have not yielded acceptable results. Structural reinforcement is generally ineffective against heat-induced flexure. Cooling has also been unsuccessful. Water cooling is especially undesirable because of the undesirable reactions which can occur between water and molten aluminum.
The invention described below greatly reduces or eliminates flexure in a metal distribution launder, so that accurate and repeatable measurements can be made from the launder of molten metal levels within an underlying casting station. This result is achieved without adding significantly to the cost or complexity of the launder.